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Anaudio filter is a frequency-dependent circuit, working in theaudio frequency range, 0 Hz to 20 kHz. Audio filters can amplify (boost), pass or attenuate (cut) some frequency ranges. Many types of filters exist for different audio applications includinghi-fi stereo systems, musicalsynthesizers,effects units,sound reinforcement systems,instrument amplifiers andvirtual reality systems.

Low-pass

Low-pass filters pass through frequencies below their cutoff frequencies, and progressively attenuate frequencies above the cutoff frequency.^[1] Low-pass filters are used inaudio crossovers to remove high-frequency content from signals being sent to a low-frequencysubwoofer system.

High-pass

Ahigh-pass filter does the opposite, passing high frequencies above the cutoff frequency, and progressively attenuating frequencies below the cutoff frequency.^[2] A high-pass filter can be used in anaudio crossover to remove low-frequency content from a signal being sent to atweeter.

Band-pass

Aband-pass filter passes frequencies between its two cutoff frequencies while attenuating those outside the range.^[2] A band-reject filter attenuates frequencies between its two cutoff frequencies while passing those outside the 'reject' range.

All-pass

Anall-pass filter passes all frequencies but affects thephase of any given sinusoidal component according to its frequency.

In some applications, such as in the design ofgraphic equalizers or CD players, the filters are designed according to a set of objective criteria such aspassband, passband attenuation, stopband, and stopband attenuation, where the passbands are thefrequency ranges for which audio is attenuated less than a specified maximum, and the stopbands are the frequency ranges for which the audio must be attenuated by a specified minimum. In more complex cases, an audio filter can provide afeedback loop, which introducesresonance (ringing) alongside attenuation. Audio filters can also be designed to provide gain (boost) as well as attenuation.In other applications, such as with synthesizers or sound effects, the aesthetic of the filter must be evaluated subjectively.

Audio filters can be implemented in analog circuitry asanalog filters or inDSP code or computersoftware asdigital filters.

Self-oscillation occurs when theresonance orQ factor of the cutoff frequency of the filter is set high enough that the internal feedback causes the filter circuitry to become a sine tonesine wave oscillator.

• Electrical resonance
• Electronic filter
• Equalization (audio)
• Feedback
• Linear filter
• Oscillation
• Self-resonant frequency

• Tone, EQ and filter
• Synthesizer electronics
• Electronic circuits

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